1. Field of the Invention
The invention relates to abatement of perfluorinated and hydrofluorinated compounds, in particular compounds typically used in semiconductor manufacture.
2. Discussion of the Related Art
Perfluorinated compounds (PFCs), which include CF4, C2F6, C3F6, SF6, and NF3, and hydrofluorinated compounds (HFCs), which include HCF3, are widely utilized as dry etchants in plasma processing within the semiconductor industry. These compounds are important reagents in current silicon-based processesxe2x80x94they are used extensively in plasma etching of wafers and in the cleaning of process tool-chamber interiors after plasma-enhanced chemical vapor deposition (PECVD). The semiconductor industry is experiencing significant growth in PFC/HFC consumption due to a market-driven expansion in wafer production (higher capacity fabrication lines), an increase in device complexity (higher integration from greater densities and more layers), and, due to increasingly smaller geometries, a general trend from wet (chemical) to dry (plasma) processing.
PFC/HFC-based plasma processes are typically inefficient in terms of reagent utilization, however, with actual utilization highly dependent on the specifics of process conditions and the particular PFC/HFC compounds utilized. Thus, an undesirable portion of unreacted PFC/HFC reagents tend to be emitted as process effluents. Unfortunately, PFCs/HFCs exhibit extremely long atmospheric residence-times and are also strong infrared absorbers. For these reasons, the compounds are believed to contribute to the atmospheric greenhouse effect and global warming. Such concerns have prompted PFC/HFC suppliers, industries that use or generate PFCs/HFCs, and environmental regulatory agencies to monitor and restrict levels of PFC/HFC atmospheric emissions. Numerous control options are under development in an effort to devise an environmentally-benign and cost-effective approach to reduction of atmospheric PFC/HFC emissions.
Some form of abatement technology is expected to constitute the first-line option for PFC/HFC emissions reduction at the IC manufacturing facility level. While various thermal-combustion, catalytic-combustion, and plasma-based PFC/HFC destruction systems are under development, improved abatement systems are actively being sought.
The invention provides an improved point-of-use PFC/HFC abatement process involving an electrochemical route to abatement. According to the invention, PFCs/HFCs are treated with a relatively strong chemical reducing agent, typically an alkali metal such as lithium, potassium, or sodium, in a system that promotes ionization of the alkali metal, e.g., by inclusion of ammonia or other compound that promotes dissolution of the alkali metal to ions. The electrochemical potential arising from the ionization process provides sufficient energy to extract the fluorine atoms from the PFCs/HFCs. The system also contains a weak acid proton donor to provide a hydrogenation source. Specifically, the weak acid is believed to displace the reaction equilibrium by way of the alkali metal, to allow the reaction to proceed at a sufficient rate. And continuous Fxe2x88x92 removal via formation of insoluble alkali metal-F precipitates (e.g., NaF) appears to provide the driving force to achieve formation of successively, and completely, reduced species. In particular, decomposition of fluorocarbon PFCs/HFCs is believed to occur primarily by fission at the carbon-halogen bond with subsequent proton addition, thereby generating a carbon-hydrogen bond. (See, e.g., FIG. 1.) Typically, this abatement system is contained in a fluidized bed to promote effective mixing, i.e., gas-liquid intermingling, of system reagents, which provides desirable reaction efficiency.
The weak acid is typically an alcohol (particularly an aliphatic alcohol) or an amine. Ethanol and t-butanol have been found to be useful. (Weak acid indicates a pKa between 14 and 22.) Strong acids would be expected to undesirably promote hydrogen evolution as a side reaction, which would reduce reduction efficiency, and thus the weak acid is believed to provide desirable properties. Sodium is an advantageous reducing agent, with contemplated sodium-containing compounds including NAK (sodium potassium alloy), sodium biphenyl, sodium borohydride, sodium benzophenone, as well as molten metal technology such as molten sodium dispersions. Lithium analogs of these sodium compounds are also possible. Suitable dissolution agents, in addition to ammonia, include low molecular weight aliphatic primary or secondary amines, where low indicates less than 10 carbon atoms. Examples include ethylamine, methylamine, isopropylamine, ethylenediamine, and butylamine. A typical reducing agent/dissolution agent/weak acid system, particularly suitable for fluorocarbon PFCs/HFCs, is sodium/ammonia/ethanol.